Electro-wetting on dielectric (EWOD) is a well-known technique for manipulating droplets of fluid by application of an electric field. It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 72, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of electrodes 38 (e.g., 38A and 38B in FIG. 1) are realized. The electrode of a given array element may be termed the element electrode 38. A liquid droplet 4, may constitute any polar (or partially polar) material (which is commonly also aqueous and/or ionic), and is constrained in a plane between the lower substrate 72 and a top substrate 36. A suitable gap between the two substrates may be realized by means of a spacer 32, and a non-polar fluid 34 (for example an oil, for example dodecane or silicone oil or any other alkane or mineral oil) may be used to occupy the volume not occupied by the liquid droplet 4. Alternatively, and optionally, the volume not occupied by the liquid droplet could be filled with air. An insulator layer 20 disposed upon the lower substrate 72 separates the conductive electrodes 38A, 38B from a first hydrophobic surface 16 upon which the liquid droplet 4 sits with a contact angle 6 represented by θ. On the top substrate 36 is a second hydrophobic layer 26 with which the liquid droplet 4 may come into contact. Interposed between the top substrate 36 and the second hydrophobic layer 26 is a top substrate electrode 28.
The contact angle θ 6 is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-liquid (γSL), liquid-gas (γLG) and non-polar fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ              SG                        -                          γ              SL                                            γ            LG                                              (                  equation          ⁢                                          ⁢          1                )            
In certain cases, the relative surface tensions of the materials involved (i.e. the values of γSL, γLG and γSG) may be such that the right hand side of equation (1) is less than −1. This may commonly occur in the case in which the non-polar fluid 34 is oil. Under these conditions, the liquid droplet 4 may lose contact with the hydrophobic surfaces 16 and 26, and a thin layer of the non-polar fluid 34 (oil) may be formed between the liquid droplet 4 and the hydrophobic surfaces 16 and 26.
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. element electrodes 38, 38A and 38B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic surface 16. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different element electrodes (e.g. 38A and 38B), the liquid droplet 4 may be moved in the lateral plane between the two substrates 72 and 36.
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 6,565,727 further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials.
U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in active matrix (AM) display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electro-wetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based electronics to control an EWOD array, namely:                Driver circuits can be integrated onto the AM-EWOD array substrate.        TFT-based electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require EWOD actuation voltages in excess of 20V to be applied.        
A disadvantage of U.S. Pat. No. 7,163,612 is that it does not disclose any circuit embodiments for realizing the TFT backplane of the AM-EWOD.
EP2404675 (Hadwen et al., published Jan. 11, 2012) describes array element circuits for an AM-EWOD device. Various methods are known for programming the array and applying an EWOD actuation voltage to the EWOD element electrode. The voltage write function described includes a memory element of standard means, for example, based on Dynamic RAM (DRAM) or Static RAM (SRAM), and input lines for programming the array element. Whilst EWOD (and AM-EWOD) devices can be operated with either DC or AC actuation voltages, in practice there are many reasons for preferring an AC method of driving, as reviewed in the previously cited reference R. B. Fair, Microfluid Nanofluid (2007) (3:245-281). It may be noted that droplets can be actuated and manipulated for a wide range of AC driving frequencies ranging typically from a few hertz to several kHz.
A method for implementing an AC driving method in an AM-EWOD device is to apply a ground potential to the top substrate electrode 28. To program a given element electrode in the array to a non-actuated state, the element electrode is written to a ground potential. To program a given array element electrode 38 to an actuated state, the element electrode potential 38 is written to have a potential that alternates between VEW and −VEW. However this method of driving has the significant disadvantage that the maximum voltage that must be switched by the transistors in the circuit in order to drive the element electrode 38 is required to be 2VEW.
U.S. Pat. No. 8,173,000 (Hadwen et al., issued May 8, 2012) describes an AM-EWOD device with array element circuit and method for writing an AC actuation voltage to the electrode. The AC drive scheme described by this patent utilizes the application of AC signals to both the element electrode 38 and to the top substrate electrode 28 of the device. Therefore, the device is capable of generating an electro-wetting voltage (voltage between the element electrode and the top substrate electrode 28) that varies between +VEW and −VEW, whilst the transistors in the array element circuit are only ever required to operate with a rail-to-rail voltage of VEW.
Many applications of EWOD technology require that the temperature of liquid droplets be controlled and/or varied. Examples include molecular diagnostics, material synthesis and nucleic acid amplification. A number of approaches have been taken to providing temperature control in a microfluidic device. One approach to achieving thermal control is to control the temperature of the entire device and its housing by external means, e.g. a hot plate. This suffers from the disadvantages that the rates of temperature change that can be achieved are generally low, and that a long time is required for the whole arrangement to reach thermal equilibrium. A number of other approaches to address this problem have been described.
U.S. Pat. No. 7,815,871 (Pamula et al, issued Oct. 19, 2010) discloses a droplet microactuator system incorporating an EWOD device with one or more heating zones for temperature control.
U.S. Pat. No. 8,459,295 (Kim et al, issued 11 Jun. 2013) discloses a microfluidic device for droplet manipulation according to the EWOD principle, wherein one or more of the electrodes on the bottom substrate comprises a heating element in the form of a patterned electrode.
U.S. Pat. No. 8,339,711 (Hadwen et al, issued Dec. 25, 2012) discloses an AM-EWOD device, with heater elements realized in the same conductive layer that is used to control droplet motion.
US20130026040 (Cheng et al, application published Jan. 31, 2013) discloses a microfluidic platform comprising an AM-EWOD device with an active matrix array of independently addressable heating elements, which may be formed in the same or different substrates, above or below a droplet handling area. This arrangement provides for independent actuation and heating of liquid droplets.
Each of these approaches has disadvantages, with many of them involving multiple layers of patterned material that must be aligned with one another, adding complexity and cost to the manufacturing process. This is an important consideration for Lab on a Chip applications, particularly where the chip must be disposable for reasons such as biological or chemical contamination of the surfaces by the reagents and samples that are used.